1. Field of the Invention
The invention is based on a method for zone heat treatment of a metallic workpiece.
2. Discussion of Background
Workpieces are zone heat-treated in order to obtain certain crystal structures orientated in a preferred direction. In this heat treatment, the workpiece with a clearly defined longitudinal axis, in which longitudinally oriented columnar crystals are to be generated, is usually displaced along the longitudinal axis relative to a heat source. Fundamentally, it is possible to distinguish between three different conventional types of zone heat treatment methods:
(A) Feeding the workpiece through an induction coil (inductive heating); this produces a large temperature gradient in the longitudinal direction. The temperature/feed diagram has a curve shape with steep flanks before and after the induction coil and a narrow, sharp maximum at their apex (see DE-A-No. 2 219 275). PA1 (B) Passing the workpiece through a furnace with a length large in comparison with the transverse dimension of the workpiece; this causes a comparatively small temperature gradient in the longitudinal direction. The temperature/feed diagram has a curve shape with flat flanks at the inlet and outlet from the furnace and a flat wide maximum between them (see Cairns et al., Met. Trans. A 6A, 1975, 179). PA1 (C) Vertical immersion of the workpiece in a salt bath; this causes a single-side temperature gradient on the inlet side. The temperature/feed diagram has a curve shape with a medium slope which flattens out, followed by an approximately horizontal part at the maximum workpiece temperature.
The methods mentioned above are generally only suitable for small workpieces with a cross-section of a few square centimeters. In the case of larger workpieces, the necessary temperature gradient in the longitudinal direction cannot be achieved and the feed velocities are too low for economic manufacture.
There is, therefore, a need for an improvement to these zone heat treatment processes.